The present invention relates generally to the contouring of optical surfaces. More specifically, the present invention relates to devices, systems, and methods for contouring optical surfaces with laser beams. Merely by way of example, devices systems and methods of the present invention are described with reference to the treatment of eyes during photorefractive keratectomy (PRK) and the like. The devices, systems and methods of the present invention can be used with other optical contouring applications, for example, the fabrication of spectacles and contact lenses.
Known laser eye surgery procedures generally employ an ultraviolet or infrared laser to remove a microscopic layer of stromal tissue from the cornea of the eye. The laser typically removes a selected shape of the corneal tissue, often to correct refractive errors of the eye. Ultraviolet laser ablation results in photodecomposition of the corneal tissue, but generally does not cause significant thermal damage to adjacent and underlying tissues of the eye. The irradiated molecules are broken into smaller volatile fragments photo-chemically, directly breaking the intermolecular bonds.
Laser ablation procedures can remove the targeted stroma of the cornea to change the cornea's contour for varying purposes, such as for correcting myopia, hyperopia, astigmatism, and the like. Control over the distribution of ablation energy across the cornea may be provided by a variety of systems and methods, including the use of ablatable masks, fixed and moveable apertures, controlled scanning systems, eye movement tracking mechanisms, and the like. In known systems, the laser beam often comprises a series of discrete pulses of laser light energy, with the total shape and amount of tissue removed being determined by the shape, size, location, and/or number of laser energy pulses impinging on the cornea. A variety of algorithms may be used to calculate the pattern of laser pulses used to reshape the cornea so as to correct a refractive error of the eye. Known systems make use of a variety of forms of lasers and/or laser energy to effect the correction, including infrared lasers, ultraviolet lasers, femtosecond lasers, frequency multiplied solid-state lasers, and the like. The lasers of these laser systems typically deliver a series of laser beam pulses during a treatment.
Known corneal correction treatment methods have generally been successful in correcting standard vision errors, such as myopia, hyperopia, astigmatism, and the like. By customizing an ablation pattern based on wavefront measurements, it may be possible to correct minor aberrations so as to reliably and repeatedly provide visual acuity greater than 20/20. Such detailed corrections will benefit from an extremely accurate ablation of tissue.
With many laser ablation procedures, the epithelium is generally removed so that the permanent optical correction can be ablated into the stroma and/or Bowman's membrane. With PRK the epithelium is removed to expose Bowman's membrane and/or the stroma. Epithelial removal has been accomplished mechanically and with laser ablation of the epithelial layer. Mechanical removal of the epithelial layer can be accomplished with mechanical scraping of the epithelial tissue layer to expose Bowman's membrane and/or the stroma. Another mechanical approach is to remove the epithelium with a brush. With Laser-Assisted Sub-Epithelial Keratectomy (LASEK), the epithelial layer is removed from the cornea as a sheet so that the layer can be replaced following the ablation of stromal tissue. Although these mechanical methods of epithelial removal have been successful clinically, mechanical removal of the epithelium takes time and can be perceived by the patients as invasive because the surgeon will touch the front surface of the eye with surgical instruments. Even though topical anesthesia is often applied to the cornea so that the patient cannot feel the surgeon touching his or her cornea, the patient can become nervous while the surgeon touches the front surface of the eye with the instruments, possibly delaying the procedure.
Laser ablation of the epithelium, also referred to as trans-epithelial ablation, can be less invasive and faster than mechanical approaches to removal of the epithelium. However, work in connection with the present invention suggests that the known methodologies for laser ablation of the epithelium may be less than ideal, and in some instances the epithelial layer may not be ablated uniformly. Thus, a surgeon will often mechanically scrape the epithelium after laser removal of the epithelium to ensure that no residual epithelial debris remains before ablating stromal tissue.
In light of the above, it would be desirable to provide more accurate trans-epithelial ablations over large areas of the cornea, for example customized trans-epithelial ablations, while avoiding at least some of the limitations of known systems.